While things have changed over the years, I have sold posters both wholesale and retail for many years. A poster can be on heavy or flimsy paper. It will obviously cost more for better papers. I don't know how you expect to market these posters. Some of the local events in my area publish posters. They sell them at the event and often for years afterward as fundraising for non profits. Some people collect them for every year as decoration for their homes. Nevertheless on average they are short print runs.
A poster can be almost any size. If you wish to be cost conscious in many ways a standard size may be something to take into account. Mostly done to eliminate custom framing costs. It's not unusual for a poster to cost $200 or more for custom framing. You might wish to make some arrangement with a local framer. Most poster publishers have no regard for this however. Some well designed posters or posters using artists who gain some reputation appreciate in value dramatically but that is rare. For example I recently noticed a poster that I remember selling for somewhere in the $30 to $40. range in the 80's or 90's that is now going for $500. But that is extremely rare and makes me feel old.
If you are targeting only the 10,000 visitors I would not expect to sell very many and printing costs can be higher than most folks expect. A short run of prints done offset can be thousands of dollars no guarantee that they all sell or that you recoup your expenses. I'd look into some form of short run digital printing.
John is a a bit low on his price estimate I believe. it's not unusual for posters to retail for $25. or $35. each depending on size, papers and printing technology. $10. seems to me the exception not that you can't go there. Check out sites like art.com which sells 100,000's of posters and limited editions both. I'm not saying you can't sell them for less but you are trying to make some money aren't you? Maybe you wish to make it a limited edition (signed and numbered) for more money each or even some portion signed by the artist for a premium price, the rest of the run at a lower poster price.
I'd offer them framed, unframed, signed, unsigned, to hit as many price points as you can at the event.
If you are selling to the mass market, not just this event you will have to price your prints so that you can make a profit while selling wholesale. In that it is not unusual to have to offer 50% off then another 50% off that and even another 20% more. For this to work out you must sell hundreds of prints at a clip and you have to mark up about 8 times your cost to make any money selling them steeply discounted.
A poster can be digital, offset lithography or serigraph (silk screen). Each will have different costs of production and therefore different retails. I hope this is helpful.
